Almost working from 1st try

gridpoint.lua

@@ -0,0 +1,31 @@
+require 'point'
+require 'config'
+
+-- Point table with grid-based coordinates
+---@class GridPoint: Point
+GridPoint = Point
+
+-- Convert grid x, y to global x, y
+---@return Point
+function GridPoint:absolute()
+    return Point:new(
+        self.x * Config.cellSize + Config.gridOffset,
+        self.y * Config.cellSize
+    )
+end
+
+-- Same as coords, but converted to global coords
+---@return number, number
+function GridPoint:globalCoords()
+    return self.x * Config.cellSize - Config.cellSize / 2 + Config.gridOffset,
+    self.y * Config.cellSize - Config.cellSize / 2
+end
+
+-- Returns local coords from global
+---@param point Point
+---@return GridPoint
+function GridPoint.snapCoords( point )
+    local x = math.ceil( (point.x - Config.gridOffset) / Config.cellSize )
+    local y = math.ceil( point.y / Config.cellSize )
+    return GridPoint:new( x, y )
+end